Conventional photomultiplier tubes include a vacuum envelope containing a photocathode, several dynodes and an electron collector. Light entering the tube through a window and incident on the photocathode causes electrons to be emitted by the photocathode. The electrons impinge on the successive dynodes, causing electron multiplication by secondary emission. After impingement on the last dynode, the electrons are collected and delivered on an output lead of the tube to provide an output signal which is representative of the input light.
A hybrid photomultiplier tube includes a photocathode, electron focusing electrodes and an electron bombarded photodiode anode. Electrons emitted by the photocathode are focused onto the photodiode. The electrons penetrate into the photodiode material and create electron-hole pairs, causing a multiplication effect. Gain is produced by the photodiode rather than the dynodes, as in the conventional photomultiplier tube.
A hybrid photomultiplier is disclosed by L. K. van Geest et al in "Hybrid Phototube With Si Target", SPIE, Vol. 1449, Electron Image Tubes and Image Intensifiers, II, 1991, pages 121-134. A photomultiplier tube using both dynodes and an impact ionization diode for electron multiplication is disclosed in U.S. Pat. No. 3,885,178 issued May 20, 1975 to Goehner.
A bias voltage on the order of 10 kilovolts is typically applied between the anode and cathode of a hybrid photomultiplier tube. The electrons are accelerated by the applied field and bombard the photodiode anode, which results in multiplication gain. However, electron bombardment at the photodiode surface can generate positive ions due to surface contamination resulting from intermediate processing steps as well as mobile cesium atoms on the diode surface due to cathode and tube body cesiation. In particular, tube body cesiation just prior to cathode installation or seal is a necessary requirement to extend cathode life and tube operation and hence there is a concomitant contamination of the diode surface by cesium. The contamination of the diode surface can be positively ionized by energetic photoelectrons striking the surface. These ions are then accelerated by the applied tube bias of 10 kilovolts in a direction opposite to the photoelectrons, by the positive nature of their charge, and hence toward the light sensitive photocathode. These accelerated positive ions strike the photocathode surface and damage the enabling cesium oxide layer formed during the photocathode cesiation process or the underlying photocathode material resulting in reduced operational life of the photocathode. In addition, positive ions striking the photocathode surface results in a pulse of electrons emitted into the vacuum envelope which are accelerated to and multiplied at the photodiode anode. This results in random output pulses at the anode proportional in average number per unit time interval to the incoming light intensity on the photocathode. Therefore detector noise performance is also degraded.
One prior art approach to eliminate the deleterious effects of ion feedback in vacuum tubes is the ion trap which usually consists of one or more electrodes placed near the collecting anode and the application of positive bias on these electrodes. The geometry of these ion trap electrodes is designed not to interfere with the electron trajectories or orbits throughout the vacuum tube and therefore the general performance of the vacuum tube. The application of sufficient positive bias to the ion trap electrodes can divert or trap positive ions generated at the anode by incoming energetic electrons. The positive ions are therefore deterred or prevented from entering the bulk of the vacuum tube where they can be accelerated and damage or degrade other vacuum tube components including the cathode. A significant difficulty with this approach is related to the required positive bias of the ion trap which requires one or more additional electrical feedthroughs through the vacuum envelope to access the ion trap electrodes. Furthermore, many vacuum tubes operate with one polarity bias supply, usually negative for grounded anode configuration; however, the inclusion of an ion trap as described requires a positive bias and therefore a bipolar power supply to operate the vacuum tube properly. This adds to the expense and difficulty of operating the device.